[XENBUS] Don't perform normal state-machine transitions and

[XENBUS] Don't perform normal state-machine transitions and
notifications during shutdown. When rootfs goes away this can
cause the system to hang, as we notify userspace etc.
Instead we forcibly reset the state machine back to state Closed
if we are a frontend driver, to get the backend to release
resources. Necessary if we are going to kexec.
Signed-off-by: Keir Fraser <keir@xensource.com>